CA1123512A - System for damping vibrations in a deflectable transducer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Electronic damping of a deflectable videotape transducer is effected by apparatus including a deflectable support arm for supporting the transducer and deflecting it in response to an electrical deflection signal. A signal transducer generates a deflection velocity signal representa-tive of instantaneous deflection velocity of the transducer.
A feedback loop receives the deflected velocity signal, converts it to a damping signal, and applies the damping signal to the deflectable support arm to dampen vibrations therein.

Description

~35~L~

This invention is ~irecteci to an electronic damping system for use with a deflectable reproduce or "read" transducer assembly which "reads" information previously recorded electronica7ly on a recording medlum such as a videotape.
Systems for sensing the position of a read transducer in a videotape recorder and for using the position information to align the transducer with the track bein8 read are described~ for example, in commonly assigned co-pending Applications by Richard Allen Hathaway, Serial ~o. 274,284, entitled POSITIONABLE T~ANSDUCE~
MOUNTING STRUCTURE and Serial No. 274,280, entitled AUTOMATIC SCAN TRACKING, both filed on March 18, 19777 and by David Edward Brown, Serial NoO 274,3689 entitled TRANSDUCER ASSEMBLY VIB~ATI~N SENSOR, filed on March 21, 1977. Brieflyg the systems disclosed in those applications eense the position of the read transducer with respect to a magnetic track, develop a correction signal when the transducer is not aligned with the center of the track, and apply the correction signal to an electrically deflectable member upon which the transducer is mounted.
The deflectable member responds to the correction signal by deflecting in an amount so as to align the transducer with the center of the desired track. In one embodiment of these systems, the deflectable member is a pieÆo-electric bimorph which is cantilevered at one end portion and supports the read transducer at its free deflectable end portion.
In some videotape re-corders, the read transducer physically touches the vid~otape when it is reading a track. However, the transducer also loses contact with the tape briefly at the end of a track and resumes contact mb/ - 2 -~3~

with the tape at the beginning of a new track. As the transducer makes and loses contact with the tape, a mechanical impulse is received by the transducer. In other videota~e recorders, such as most helical type videotape recorders, a thin film of air is established between the transducer and the tape that keeps the transducer out of contact with the tape as it scans the tape. The transducers of these v~deotape recorders also experience an impulse as they enter and leave the scan of the tape because the coefficient of friction of the air fiIm is several orders of magnitude greater than that of the free space conditions encountered by the transducers between scans of the tape. In the case where the transducer is mounted at the end of a bimorph, the impulse will cause the bimorph to vibrate or oscillate and cause the transducer to overshoot its proper position.
While so-called dead-rubber pads positioned on either side of the transducer to absorb impact without immediate rebound may be used, such pads limit the amount of controlled deflection which can be otherwise effected and therefore restrict the dynamic deflection range of the transducer. In addition, if such pads are used in a helical videotape recorder where the scanning drum which carries the transducer head assembly rotates at a high speed, the pads are subjected to forces which may apprnach or even exceed 1,000 G's. At this level of force, it is difficult to ensure that the pads remain in their assigned positions.
Vibrations in a read transducer can be generated by electrical impulses as well as by mechanical impulses.
For example, in a videotape recorder such as that described in Application by Richard Allen Hathaway et al, ~ mb/ - 2a -;

~3~

Serial ~o. 274,370, entitled M~THOD AND APPARATUS FOR
PRODUCING SPECIAL MOTION EFFRCTS IN VIDEO RECORDING
AND REPRODUCING APPARAT~S, filed on even date herewith, slow motion and other effects in a reproduced video scene can be produced. For example, a half speed slow motion playback can be produced by reducing the tape transport speed to one-half its normal speed and by causing the read transducer to read each track twice.
In order to repeat the playbac~ of the track, i.e., play back the track the second time, the read transducer must be physically repositioned or reset to the beginning of the track which is to be repeated. This resetting of the read transducer is accomplished in one embodiment of the recorder disclosed in the Hathaway et al application by applying an electrical reset signal to a deflectable bimorph upon which the read transducer is mounted to thereby deflect the bimorph and the read transducer to reset the transducer to the beginning of the desired track. The reset signal is in the form of an electrical impulse which may tend to cause the bimorph to vibrate or even oscillate at its resonant frequency and, as polnted out above, such vibrations must be damped to ensure correct alignment between the read transducer and the videotape. The use of dead-rubber pads to effect the damping of electrically induced oscillations poses the same difficulties referred to above with respect to the damping of mechanically induced oscillations.
Accordingly, it is a general object of the present invention to provide an improved device for damping vibrations in a deflectable transducer.

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~ rnb/~ 3 It is a ~o~e specific object of this invention to provide a damper which neither restricts the dynamic range of a deflectable transducer nor is subject to the noted deficiencies of mechanical dampers under high G forces.
Thus, the present invention provides damping apparatus for damping vibrations in a deflectable transducer, comprising: a transducer support arm Eor suppor~ing a transducer and for deflecting the transducer in response to an applied electrical deflection signal;
signal generating means for generating a positive signal representative of the instantaneous defleeted position of the transducer; a negative feedback loop coupled between the OUtpllt of the signal generating means and the support arm and including means for converting the position signal to a damping signal and for applying the damping signal to the support arm to dampen vibrations therein; and means for adding a portion of the applied electrical deElection signal to the position signal to reduce the gain of the feedback loop at frequencies near the first order anti-resonance of the transducer.
In a further aspect, the invention provides damping apparatus for damping vibrations in a deflectable videotape transducer, comprising: a transducer support arm for supporting a videotape transducer and for deflecting the transducer in response to an applied eleetrical deflection signal~ the deflectable support arm comprising a piezoelectric bimorph having first order resonance characteristics and higher order resonance characteristics; signal generating means for generating a deflection velocity signal representative of the :

instantaneous deflecting velocity of the transducer;

mb/ ` ~` - 4 -. . .

3~

a negative feedback loop coupled between the output of the signal generating means and the support arm and inclllding means for converting the deflection velocity signal to a damping signa]. and means for applying the damping signal to the deflectable support arm to dampen vibrations therein, the feedback loop :including a filter for excluding from the damping signal the component of the deflection velocity signal attributable to the higher order resonance characteristics.
The features and advantages of the present ., lnvention will become apparent upon reading the following detailed description, while referring to the attached drawings, in which:
FIG. 1 is a perspective view of a portion of a helical videotape recorder simplified for the sake of clarity and particularly illustrating a rotatable scanning drum and read head;
FIG. 2 is a perspective view of a read transducer assembly for use with the read head of FIG. 1, mb/ ~ a -Z
I.D. 2519 FIG. 2a is an enlarged cross-section of a portion of the transducer assembly shown in FIG. 2 and illustrating the layered construction of the assembly;
FIG. 3 is a block diayram of a feedback cont:rol system embodying various aspects of this invention for con-trolling vibrations in a bimorph read transducer assembly;
FIGS. 4a and 4b graphically illustrate -the frequency and phase response of the bimorph transducer assembly used in the control system of FIG. 3;
FIG. 5 is a schematic diagram of the control system illustrated in FIG. 3;
FIG. 6 shows prior art methods of cleflecting a bimorph; .
FIG. 7 illustrates an improved method of deflecting -~
a bimorph;
FIG. 8a illustrates an improved method of varying the direction and magnitude of deflection of a bimorph;
: FIG~ 8b graphically illustratesthe net voltage which 1s applied to one element of the bimorph shown ~ ln FIG. 8a;
FIG. 9 shows an improved method of driving a bimorph when the bimorph deflection signal does not include very low frequency or DC components;
FIG. 10 is a schematic diagram of a deflectable read transducer system embodying the improved bimorph de-flection method illustrated in FIG. 8a.

~3~

FIGURE llis a plan view of a magnetic head drum for he~.ical tape recording use~, showing thc invention mounted thereon;
FIGURE 12is an exploded perspective view, to an englaryed scale, of a portion of the str.ucture shown in FIGURE ll.
FIGURL' 13is an er-larged sectional view taken on ~ the plane of lines13-13Of FIGUREll.
:; lO FIGURE 14is a left~end elevation view of a portion of the structure shown in FIGURE 13.
FIGURE 15is a right-end elevation viet~ of a portion of the structure shown in FIGURr. I3.
FIGURE 16is an enlarged fragmentary perspective view illustrating a portion of the structure shown in FIGURE 13.
; FIGURE 17is an enlarged left-end view of a portion of the structure shown in FIGURE13 illustrating an arrange-ment of a plurallty of transducers thereon.
FIGURE 18is an elevation view of a portion of tape;
FIGURE19 is a reduced scale view of the tape of FIGURE18 enwrapped around a scanning mechanism including the structure of FIGURE 11 a~ld FIGURE 20 is an enlarged perspect.ive view, partly in schcrnatic form, of an al~crnative cml~o~ cl~t of the invent:ion.
FIGURFS 21A and 21~ are schematic l~lock diagrams of alternati.ve embodimellts for sensing and controlling t}le position of supported transducers relative to a record surface.

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~3~
Broadly stated, this invention is directed toward apparatus for darnping vibrations in a de~lectable videotape read transducer without restricting the dynamic range of the ~0 transducer and without the adverse results expe~ienced by dead-rubber pads in the high G environmel-t of such a transducer.
The desired damping is achieved in this invention by apparatus which includes a deflectable transducer support arm and a signal generator for generating a signal repre-sentative of instantaneous deflection velocity of the arm.
A negative feedback loop is coupled between the output of the signal generator and the support arm for converting the deflection velocity signal to a damping signal. The damping signal is applied to the transducer support arm to damp its vibrations.
In a preferred embodiment of this invention, the generator which produces the transducer velocity signal in- !
cludes a bimorph support arm having an integral sensor for developing a signal representative of the instantaneous de-flected position of the transducer~ A differentiator in the feedback loop converts the transducer position signal to a transducer velocity signal. The feedback loop also includes phase compensation means for stabilizing the loop at fre-quencies near the resonant point of the bimorph. To compen-sate for differences in responses of bimorphs at their anti-~ resonant frequencies, means are included for artificiallyadjusting antiresonant frequency since loop stability relies on it being at a predictable frequency in order to maintain stability.

35 ~
- It should be apparent from the following description that this invention is useful in a variety of applications and is particularly useful in the environment of a helical video-tape recorder. Accordingly, while the embodiments of the invention specifically shown and described herein are in con-junction with helical videotape recorders, it should be under-stood that the scope of the invention should not be limited to such helical recorders.
To more clearly describe this invention and its co-operation with other inventions which find use in helical videotape recorders, this desciption will cover not only embodiments of this invention but also embodiment of o.her inventions not claimed herein. The inventions described but not claimed herein are claimed in commonly assigned co-pending Applications by David Edward ~rown, Serial No.
274,368, entitled TRANS~UCER ASSEMBLY VIBRATION S~NSOR, and by Raymond Francis Ravizza, Serial No. 274,424, entitled DRIVE CIRCUITRY FOR CONTROLLING MOV~BLE VIDEO HEAD, filed on even date herewith. As has been pointed out above, some of the embodiments described herein relate to helical video tape recorders. They are particularly related to apparatus and methods for controlling the alignment of a read trans-ducer with respect to tracks on a videotape. Therefore, a brief description of the operation of a videotape recorder read transducer will first be given.
Referring to the drawings, and particularly FIG. 1, a scanning drum 20 of a helical videotape recorder is shown and has a rotatable portion which carries a reproducing or "read" head which contacts and scans successive tracks on a magnetic videotape.

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The scanni.ny drum 20 has a pair of drum portions 22 and 24 around which a videotape 26 is wrapped. The tape 26 is : .

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caused to move by tape transport means (not shown) in the di-rection of the arrows A and wraps around the drum portions 22 and 24 in a helical pathO The tape 26 is kept in tight contact and alignment with the drums by guide rollers 28 and 30 and by tension exerted on the tape by the tape transport.
In a helical videotape recorder the information tracks run diagonally with respect to the lengthwise dimension of the tape, and a portion of one such track 32 whose size is exaggerated for clarity is shown in FIG. 1. In order to sense the information recorded on track 32 a read transducing head 34 is mounted on drum portion 22 which rotates in the direction of the arrow B.
- The movement o~ the tape 26 and the rotation of the transducer 34 causes transducer 34 to contact the tape along the track 32 and to generate an electrical signal repre~sentative of the information previously recorded on the track. This electrical signal is fed to signal processing circuitry for processing in a manner well known in the art.
It is apparent that the extent to which the transducer 34 can faithfully reproduce the information originally recorded on the track 32 depends on the transducer 34 accurately following or tracking the track 32. Tracking problems arise, for example, when videotapes or the tracks become distorted, as by temperature or humidity induced dimensional changes in the tape, or by faulty tensioning mechanisms in the tape transport, for example.
Because of such trac]cing problems, it is desirable to sense the instantaneous position of transducer 34 with respect to track 32. Apparatus for sensing the position of a read trans-ducer with respect to a track is disclosed and claimed in 3L~;23Sl;~:

the aforementioned ~athaway application, Serial No.
274,280. Briefly, when perfect tracking between the read transducer and the track is not occurring, an electrical correction signal is applied to a deflectable support arm such as a bimorph on which the read transducing head is mounted~ The correction signal causes the support arm to deflect the transducer toward track center and thus reduces tracking errors.
Deflection of the read transducer is also desirable in helical videotape recorders such as that described in the aforementioned Hathaway et al application, Serial ~o. 274,320, wherein slow motion and other effects in a reproduced video scene are generated, the approximately half speed slow motion effect, for example being produced by reducing the tape transport speed to one-half its normal speed and by causing the read transducer to read each track twice. In order to read a track twice, the read transducer must be , physically repositioned or reset to the beginning of the track which is to be repeated. This reset of the read transducer is accomplished in one embodiment of the recorder disclosed in the above noted application by applying an electrical reset signal to the deflectable support arm upon which the read transducer is mounted and thereby deflecting the support arm and the transducer so as to reset ~`

_ 9 _ jrc~

the transducer to the beginning of the desired track. The reset signal is in the form of an electrical impulse which may tend to cause the support arm to vibrate or oscillate, and such vibrations must be damped to assure correct alignmenk between the transducer and the tape.
Vibrations in the deflectable transducer support arm are also lnduced when the transducer makes and loses contact 23~

with the tape. For example, in the scanning drum arrangemen-t of FIG. 1, read transducer 34 experiences a dropout because it loses contact with tape 26 in the gap between guide rollers 28 and 30 during each rotation of the drum 20. Contact between the transducer 34 and the tape 26 is re-established as transducer 34 passes roller 28 as it rotates in the direction of arrow B.
The vibrations set up in a deflectable transducer support arm are, of course, undesixable since they can produce a loss of tracking. This loss of tracking due to vibrations can be minimized or eliminated by sensing the vibrations in the ~eflectable support arm and applying a damping signal to the suppoxt arm to counteract the vibrations.
Thus, in helical videotape recorders in which it is desirable to include a deflectable support arm for reducing tracking errors, it is also desirable to include means for damp- -ing electrically and mechanically induced vibrations in the deflectable support arm. Preferably, d~mping the vibrations can be done electronicall~, in which case some means for sensing the amplitude of the vibrations and for generating an electrical signal indicative therebf, is required.
A deflectable read transducer assembly which includes means for sensing vibrations induced therein is shown in FIG. 2 and is indicated generally by reference numeral 36.
At one end of assembly 36 is the read transducer 34 itself. Its output is coupled via wires 38 to a pair of trans-ducer output terminals 40 from which the transducer output is fed via line 82 to a conventional video processin~ circl~it 84-A support arm, indicated generally at 42, for holdingand deflecting transducer 34 is a piezoelectric bimorph which -lOa-3~

deflects or bends when a deflection potential is applied to it. The bimorph is formed from a number of layers bonded together to act as a piezoelectric motor 43 and includes a top piezo-ceramic element or layer 44 and a bottom piezo-ceramic element or layer 46. The various layers of transducerassembly 36 are shown more clearly in FIGo 2a. Piezo~ceramic elements 44 and 46 are both bonded to a common, electrically conductive substrate 48. Substrate 48 limits the movement of the bimorph to a bending motion in response to an applied electrical potential.
In order to impress an electrical potential to piezo ceramic elements 44 and 46, conductive layers 50 and 52 cover the outer surfaces of elements 44 and 46. Terminals 54 and 56 (FIGo 2) are electrically connected to layers' 50 and 52, respectively, for re¢eiving an electrical deflection potential.
Substrate 48 also has an input terminal S8 to serve as electrical common for the applied deflection potential. The electrical potential for deflecting support arm 42 is applied across piezo-ceramic element 44 between terminals 54 and 58 and across piezo-ceramic element 46 between terminals 56 and 58.
In order to force suppor~ arm 42 to deflect at its free end 60 where transducer 34 is mounted, arm 42 is canti-levered between insulating spacers 64 which may be held in place by a bolt (not shown) passing through hole 66.
In operation, appropriate deflection potentials are applied across piezo-ceramic elements 44 and 46 via input ~23~

terminals 54, 56 and 58. Support arm 42 then bends at its free end 60 and deflects transducer 34 in a direction and to an ex-tent which is dependent on the magnitude and polarity of the potentials applied to termina]s 54, 56 and 58.
In some applications, a piezoelectric motor need include only one piezo-ceramic element bonded to a substrate.
For example, a single piezo-ceramic element could have a top surface covered by a conductive layer and a bottom surface bonded to a conductive substrate which forces the element to lQ bend when an electric potential is applied between the substrate and conductive layer. However, where large amounts of de-flection are required~ such as in videotape transducers, a motor element comprising two piezo-ceramic elements 44 and 46, as shown in FIG. 2j is preferable.
In addition to having a piezoelectric motor 43 for deflecting transducer 34, the assembly 36 also includes a deflection or vibration sensor in the form of a piezoelectric generator 68. The generator 68 includes, in the i]lu~trated embodiment, an edge portion 70 of the piezo-ceramic element 44 whose bottom surface is bonded to substrate 48 as previously described. It should be appreciated, however, the generator 68 could be formed by a portion located in the center~of the element 44. The generator has a separate conductive layer 72 overlying the element portions 70. The conductive layer 72 i5 isolated frorn conductive layer 50 by a dielectr~ gap 74 to electrically isolate the OlltpUt of generator 68 from potential appl~ed to the conductive layer 50.
The ~enerator 68 is cantilevered at 76 and h~s an ~2~

opposite, free deflectable end 78O Thus, whenever vibrations or deflections occur in the motor 43 due to electrical or mechanical impulses, a corresponding deflection or vibration of the free end 78 of generator element 68 occurs and produces, between the common substrate 48 and the conductive layer 72, an electrical signal indicative of the instantaneous degree of deflection of the motor 43 and of the transducer 34.
In the description of the piezoelectric motor and generator above, generator 68 was said to include a piezo-ceramic element portion 70 of the element 44 and the motor 43includes the bulk of the piezo-ceramic element 44. As shown in FIGSo 2 and 2a, piezo-ceramic element portion 7~ i.s prefer-ably part of the unitary piezo~ceramic layer or e]ement 44.
However, it is not necessary that thè portion 70 be part of a larger unitary piece. For example, gap 74 could be extended downwardly to cut through layer 44 and form a separate elemen-t 70. It has been found, however, that even with large amplitude deflect.ion signals applied to the elements 44 and 46, these deflection signals are not substantiallycoupled to generator 68 when the element portion 70 is part of the larger unitary element 44. ~Iowever, cutting the element down to the ground plane result.s in an increased isolation of the motor-to-generator and increases the element's tolerance to surace contamination.
Any vibration sensor which develops an e;ectrical output :indicative of vibrations in arm 42 should be responsive to vibrations over a frequency range extending from approxi-mately lO ~lertz up to at least 400 Hertz, at which the illustrated bimorph support arm has a resonant frequency.

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The generator 68 of Fiyure 2, by extending lengthwi.se along the support arm 42, does exhibit a yood ~reque-lcy rcsponse over tlle range desired. Tllis respot-se appears much better, particularly at low frequencies, than tlle fre4uency rcsponse of a generator which may extend transverse to the lengthwise dimension of the support arm 42.
~ rhe preferred dimensions for sup~ort arln 42 inclu~le a lenyth L extending from free end 60 to the cantilevered point ~6 of approximately 0.9 inch and a widtll W of approxi-mately O.S inch. Each of the layers 44, 46 and 43 are prefer-ably approximatel.y 0.006 inch thick while con~uctive layers 50, 52, and 72 have thicktlesses in the rallc3e of a few microns.
The wid-th of the conductive layer 72, as measured between tlle gap 74 and the nearest edge of the support arm 36, is preferably about 50 mils. The substrate 48 is preferably made of brass and the conductive layers 50, 52 and~72 are nickel depositions.
The piezo-ceramic layers 44 and ~6 are bonded to substrate 48 by an epoxy adhesive or the like.
The read transdacer assembly 36 may be enclosed in a housinc3 (not shown) haviny top and bottom portions w~lich hold assembl.y 3G betweell them. 'l'he entire housed assembly may be held to(Jctller hy a bolt pLIssin~ throuc3h appropri.ate holes it\
a top portiol, of the housilly, throuc3h hole 66 (F'~rG. 2), and throu~3h another hole in a bottom portion of the housing.
more detai.led description of a housing which may be uscd for assembly 36 is ~iven in the aforementioned Richard Allen Hathaway application, Serial No.

~14--~23t~
The pie~oelectric motor-generator combination described above is a low cost, reliable device capable of being controllably deflected and for simultaneously generating an output signal representative of the controlled deflection or of vibration-induced deflection.
It is particularly useful as part of a read transducer assembly for a videotape recorder and is illustrated schematically in connection with the videotape recorder systems described below. The piezoelectric motor-generator combination forms the subject matter of the invention described and claimed in the aforementioned Brown application, Serial No. 274,368.
The piezoelectric motor-generator combination described above which simultaneously deflects a read transducer and senses vibrations therein is used in an electronic feedback control system for damping vibrations in a videotape read transducer.
There have been transducer damping schemes which have used so-called dead-rubber pads to absorb vibrations in a transducer but the pads also limit the effective deflection range of the transducer. If the pads are mounted on the read head adjacent the transducer in a rotatable scanning drive, they are subjected to high G
forces as the drum rotates. Under these conditions, it can be difficult to keep the pads properly situated on the drum. An improved damping system in which the above-described motor-generator combination can be used is shown schematically in FIG. 3. Before describing the improved damping system, however, a brief description of associated transducer circuitry will be given in order to clearly indicate how the damping system cooperates with the associated circuitry.

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~ mb/'~''`'` - 15 ~

~3~2 Referring now to FIG. 3, a read transducer 34 operates as described above to sense or read previously recorded information in videotape tracks. The transducer 34 is part of the mb/ ~ 15a -.~ 3~
read transducer assembly 36 such as that shown in FIGURE 2 and has a de~lectable support arm 42 for deflecting transducer 34 in response to deflection signals to correct the alignment of transducer 34 with a track or to reset transducer 34 to the beginning of a track, as in the slow motion mode of operation described above. The support arm 42 is cantilevered at point 76 and its opposite end portion which supports tranducer 34 is free to deflect.
The electrlcal signal output of transducer 34 appears on conductor 82 which conducts this signal to conventional video processing circuitry 84 for generating, for example, a composite television signal ~or RF transmission.
The output of transducer 34 is also fed to a trans-ducer position control circuit 86. The function of control circuit 86 is described in the aforementioned co-pending Application by Hathaway, Serial No. 274,284 and is not a part of the present invention. Briefly, however, it describes a position control circuit 86 which generates a "dither" signal of fixed frequency for application to the deflectable support arm 42 for deflecting or "dithering"
transducer 34 back-and-forth across a track at a fi~ed rate.
Since dithering causes the transducer 34 to move trans-versely relative to the track, the signal output of transducer 34 will be amplitude modulated at the dither frequencyO The amplitude modulated signal envelope contains information concerning the alignment between trans-ducer 34 and the track being read and is detected to produce a correction signal for moving the transducer 34 toward the center oE the track. Th~s correction signal and the dither jrc:~

signal appear on conductor 88 and are ultimately applied to the deflectable support arm 42.
A transducer reset signal generator 90 develops an eleetrical signal for application to the deflectable support arm 42 for selectively resetting the transducer 34 to the beginning of a track when such is required. Circuitry for developing such reset signals is described and claimed c2 ~3-7c~
in tne aforementioned Hathaway applieation, Serial Mo. ~hr~*~~

B ~
The reset signal from signal generator 90 and the dither correction signal from eireuit 86 are both fed to a frequeney compensator 92 which comprises an amplifier whose frequency response complements the undesired residual response variations of support arm 42 when electronic feedback control damping is applied to it as shown schematieally in FIG. 3.
Frequeney compensator 92 augments the aetion of the electronic damping eireuit in order to provide the desired uniform ~requeney response for the overall system. The area of augmentation is in the 300 to 400 Hz region where the eleetronic damping action does not completely remove the rise in frequeney response of arm 42 at its first-order mechanieal resonant frequency.
The frequency compensated defleetion signals from eompensator 32 are fed via conductor 94 to a summing amplifier 96 whieh sums the deflection signals with a transdueer damping signal generated by the feedback loop described below. The output of the summing amplifier 96 is fed via eonductor 98 to a drive amplifier 100 ~hich amplifies its input and applies it to defleetable support arm 42 for controllably deflecting transdueer 34 to the center of the traek and maintaining proper transducer to traek alignment.

~3~:3L2 The various deflection signals which are applied to the support arm 42, par-ticularly signals generated by the reset generator 90, may set up unwan-ted vibrations in the arm 42. This is paxticularly true where the arm 42 is a himorph S since bimorphs exhibit resonance characteristics which tend to drive the bimorph into dam~ed oscillation.
To damp such oscillations, a negative feedback loop is included in the system shown in FIG. 3 for develop,ing an electrical damping signal and for applying the damping signal to support arm 42 to dampen its vibrations or oscillations.
The required damping signal is derived, in general, from a signal generator which generates a deflection velocity signal representative of the instantaneous deflection velocity of the read transducer 34. In the embodiment illustrated in FIG. ~, said signal generator includes a sensor 102 integral to the support arm 42 for generating a signal representaiive of the instantaneous deflected position of the transducer 34 and adiferentiator 104 for con~erting the transducer position signal to a transducer velocity signal. The sensor 102 is preferably a piezoelectric gener~tor of the type , shown in Figure 2 which is integrally formed with the bimorph support arm.
The output of sensor 102 is fed to a high input impedance amplifier 106 which presents a very small load to the sensor 102. Since the sensor 102 is typically equivalent to a voltage source in series with a capacitance, any electrical load on sensor 102 must be small in order to effectively couple low frequency signals from the sensor 102.

~~8 The outpu-t of amplif,ier 106 is coupled through a summ~r 108, whose other input will be described below, and to the differentiator 104 which differentiates the transducer position signal from the sensor 102 and converts it to a signal representative of instantaneous transducer velocity.
The diferentiator 104 is illustrated as having an amplitude versus frequency characteristic similar to that of a high pass filter and therefore introduces a phase lead to the signals it passes. The significance of the phase shift experienced by a signal traversing the feedback loop is explained immediately below in order to better appreciate the function of the remai.n.ing undescribed elements of the feedback loop.
Because the support arm 42 is preferably a piezo-electric bimorph, it exhibi.ts the well known first order resonanceand anti-resonance characteristics of piezoelectric crys-tals, as well as higher order resonance characteristics. FIG. 4a illustrates the combined frequency response of a bimorph mo-tor-generator combination of the type shown in FIG. 2. This response is generated by applying a varying frequency, constant amplitude sine wave to the piezoelectric motor and measuring the resultant output of the piezoelectric generator. The results of such a measurement are shown in FIG. 4a which indicates a resonance point near 400 Hertz and an anti-resonance point, which has been found to vary from around 700 Hertz to about 1000 Hertz, depending on the particular bimorph being used~ The maximum output of the motor-generator combination occurs at resonance and the minimum output occurs at very low frequencies and at anti-resonance.
High order resonance characteristics are not shown in FIG. 4a.

~235'~2 Si.nce the output of the motor-generator combination is ma~imum at resonance, vibrations or oscillations will tend to occur at i-ts resonant frequency when the bimorph is excited by an electrical or mechanical impulse. Therefore, to eliminate the possibili-ty of such oscillations, the feedback loop is tailored to feed back to the bimorph damping signals which are 180 degrees out of phase with respect to the signals which initially excited the bimorph into oscillation, thereby counteracting the tendency of the bimorph to oscillate.
To insure that the damping signals are of the correct phase, the phase response of the bimorph motor-generator combination must be taken into account. As indicated in FIG. 4b on the curve labeled "birnorph", signals near resonance (about 400 l~ertz) experience a phase shift of about 90 degrees in passing through the mo-tor-generator combination, and high frequency signals experience a phase shift of 180 degrees.
: In order to ensure that signals near resonance experience a net phase shift of 180 degrees around the feedback loop, and since all signals in the loop will be phase shifted 180 degrees by an inverting feedback amplifier prior to being applied to support arm 42, the signals near resonance must be phase compensated by 90 degrees so that their net phase shift is zero at the input to the inverting feedback amplifier. This insures that the loop will not oscillate at the resonan-t frequency due to instability in the feedback system. Since signals having a ; frequency far from resonance have a very low amplitude, the loop gain of the feedback loop will always be less than unity for them so that the phase shif-t which they experience will not cau5e instability in thc loop.

~235~

Returning to the feedback loop of FIG. 3, -the transducer velocity signal deve].oped by ~ifferentiator 104 is fed to a low pass filter 110 whose upper cutoff Erequency is chosen to substantially a-ttenuate signals attrihu-table to second order and higher order resonance characteristics of the bimorph.
Such signals generally have a frequency of over 2000 Hertz and are attenuated at least 20 decibels by the filter 110.
The filter 110 contributes some phase lag to signals which it passes in addition to the initial phase lag of 90 degrees due to the bimorph itself (as shown in FIG. 4bj.
To compensate for the total phase lag experienced by signals near resonance, a phase lead network 112 follows fllter 110 and shifts the phase of signals received from the filter 110 so that those signals having a frequency near .resonance have net phase shift of zero degrees upon leaving the lead network 11~. The curve labeled "with lead network"
of FIG. 4b illustrates the effect of lead network 112. In practice, the differentiator 104 also adds some phase lead and : thereby assists the lead network 112 in~properly adjusting the ~ 20 phase of the signals near resonance.
:: The signals near resonance from ]ead network 112 have a phase of zero degrees with respect to -the signals initially exciting the bimorph and are in condition to be applied to a negative feedback amplifier 114 which inverts the signals received from the lead network 112. The output of negative feedback amplifier 114 is the damping signal which is combined in the summer 96 with the transducer:
deflection signals from the conductor 94, amplified by the drive amplifier 100, and applied to the bimorph support arm 42 to damp vibrations therein. The feedback amplifier 114 has a variable amount of negative feedback for adjusting the gain of the feedback loop to accommodate differences among bimorphs.
The feedback loop illustrated in FIG. 3 also includes means for compensating for the different anti-resonance responses among bimorphs. A frequency response curve is shown by the solid line in ~IG. 4a and a dashed line indicates the variable nature of the anti-resonance characteristic among various bimorphs. For example, at 700 Hertz the frequency response of one bimorph may be considerably less than that of another bimorph, as indicated by the difference between the solid line and the dashed line at the Erequency of 700 Hertz. ~eferring to FIG. 4b, the phase response of the feedback system with the lead network is such that signals near 700 Hertz undergo a 180 degree phase shift. I~ sic3nals having a 180 degree phase shift are applied to inverting feedback amplifier 114, they will ultimately be applied to a deflectable support arm 42 in phase with the original exciting deflection signals and may lead to oscillations at that frequency if their amplitude is large enough at frequencies corresponding to positive feedback conditions for the feedback loop. simorphs having. a frequency response illustrated by the solid curve of FIG. 4a have a very small output at 700 Hertz so that the overall loop gain of the sys-tem for such signals will be low enough to avoid oscillations, irrespective of their phase response. Ilowever, bimorphs exhibiting greater gain at 700 llertz, as il]ustrated by the dashed line, may induce instability into the system if not otherwise compensa-ted for. The feedback system illus-trated in FIG. 3 compensates ~3~

for such differences between bi.morphs by adding a portion of the exciting deflection signals to the output of the sensing device 102 so that signals normally experiencing a 180 degree phase shift between their application to bimorph 42 and their output at sensor 102 will be effecti~ely nulled~
Signals experi.encing such a 180 degree phase shift are shown by FIG. 4b to be in the vicinity of anti-resonance. Therefore, signals near anti-resonance can be e~fectively nulled by . coupling across the transducer assembly 36 a portion of the signal normally fed thereto.
Referxing to FIG. 3, a means for feeding through a portion o the deflecti.on signal and combining it with the position signal developed by the sensor 102 includes the potentiometer 116 and the summer 108.~ Deflection sig,nals appearing at the output of the summer 96 are fed to both the drive amplifier lOO and the pot:entiometer 116, whereupon a portion of the deflection signals a~e fed via conductor 118 to summer 108. Summer 108 also receives, from ampliier 106, deflection position signals developed by sensor 102.
Deflection signals which undergo a 180 degree phase shift in passing through the input to support arm 42 to the output of sensor 102 (i.e. frequencies near anti-resonance) are nulled in summer 108 so that the loop is stabil.ized for frequencies near anti-resonance. This operation effectively cxeates an artificial null near 700 Hextz so that, regardless of the bimorph being used in transducer assembly 36, it will appear to have an effective nu~l near 700 Hertz so that the loop gain for signals near 700 Hertz will always be less than unity and the feedback loop will be stabilized for signals at those frequencies.

~L2~

Circuitry for effecting the functions of the various blocks in FIG. 3 is illustrated in FIG. 5.
Transducer deflection signals, including the dither signal and reset signals referred to above, are applied at terminal 120 to frequency compensator 92 which includes a pair of conventional amplifiers 122 and 124~ The frequency response of compensator 92 is shaped conventionally by the RC coupling around amplifier 122 and between amplifiers 122 and 124 to . have an overall amplification which decreases with frequency in the 300 to 400 Hz region in order to compensate for the residual Erequency-dependent variations in deflection sensitivity of support arm 42 after electronic dampiny has been applied.
The output of amplifier 1~4 is fed via co~ductor 94 to summing amplifier 96 which also receives, at its non-inverting input, an input rom the feedback control loop.
The output of summing amplifier 96 is applied to drive amplifier 100 via conductor 98.
The negative feedback loop begins at terminal 126 at which the output fro~ sensor 102 appears. The signal from sensor 102 is applied to amplifier 106 which is a conventional, frequency compensated, feedback amplifier 128. The output of amplifier 123 is fed to the inverting terminal of summing ampli~ier 108 which also receives, at the same input, a portion of the transducer deflection signals for creating the artifical null at anti-resonance as described above. Diodes 131 protect amplifier 128 from damaying high voltage transients ~-ue to accidental short circuits between sensor 102 and the input to support arm 42.
The output of summing amplifier 108 is then conducted to differentiator 104, comprising serially connected capacitor 129 and resistor 130.

, ~23~

The low pass filter 110 which receives the output of differentiator 104 is an active elliptical fil-ter com~rised of amplifiers 132 and 13~, and indicated generally at 13~.-The lead network 112 receives the output of the filter 110 and comprises a capacitor 136 serially coupled to resistor 138. The output of the lead ne-twork 112 is applied to the inverting input of a conventional feedback amplifier 114 whose feedback and therefore forward gain is varied by adjusting the variable resistor 140. The output of amplifier 114 is coup:Led to the non-inverting input of summing amplifier 96 and then applied to the drive amplifier 100 which, in turn, drives the deflectable support arm 42 for deflecting the transducer 34 in the manner previously described.
The damping system described above provides improved damping for deflectable videotape transducers wlthout restricting their dynamic range. The feedbac]~ control loop, in combination with the motor-generator transducer assembly, provides a reliable and low cost vibration damper for videotape recorders and other applications where vibrations in a deflectable bimorph transducer assembly require dampin~.
From the fore~oing, it should be understood how a videotape read transducer can be controllably deflected and damped to maintain alignment between itself and a tape track.
An improved bimorph transducer system, including a method of applying deflection signals to a deflectable bimorph to achieve maximum deflection sensitivity will now be described.
Such an improved system is useful in the tape recorder apparatus already described and will be illustrated in that environment~

.

It is understood, however, that -the improved method of driving a deflectahle bimorph disclosed below is also useful in other applications where it is desirable to achieve a large amount of bimorph deflection.
A bimorph which is used for bi-dir~ctional de1ection consists generally of two layers of piezo-ceramic material bonded to opposite sides of a conductive substrate. One end o the bimorph is cantilevered and the opposite end is left free to deflect in response to a voltage applied to the bimorph.
The direction in which a bimorph deflects depends on the polarity of the voltage applied to it and the poling direction of the pair of piezo-ceramic elements. The poling dixection of a piezo-ceramic element is established by being initially subjected to a unidirectional electric ~ield which polarizes the element accordiny to the direction of the field.
The polarized piezo-ceramic element is then said to have a "poling dixection" and thereafter exhibits unique mechanical properties when subjected to subsequently applied voltages.
A known method of causing a bimorph to deflect or bend is illustrated in FIG. 6 wherein a bimorph 142 includes ~
piezo-ceramic elements 144 and 146 bonded to opposite sides - -of conductive substra-te 148. Bimorph lq2 is canti-levered at 150 while its opposite end 152 is free to deflect.
Piezo-ceramic elements 144 and 146 are each shown with an arrow to indicate their respective poling directions.
When they are aligned as shown in FIG. 6 with their arrows pointing in the same direction, they are referred to herein as havin~ a ccmn~on poling direction.

~Z3S~;~

The poling directions sh~wn are obtained by applying a voltage across a piezo-ceramic element such that the more positive potential is at the tail of the arrow and the more n~gative potential is at the head of the arrow.
For example, in FIG. 6, bimorph 142 is shown being deflected upwardly by a voltage source 154 connected between elements 144, 146 and substrate 148. The polarity of source 154 is such that it is applying a voltage to element 144 in the same direction as its original polarizing voltage, whereas source 154 is applying a voltage to element 146 of a polarity opposed to its original polarizing volta~e. When the polarity of a deflection voltage applied to a plezo-ceramic element is identical to the polarity of that element's original polar-izing voltage, the applied deflection~voltage is referred to herein as being applied in the poling direction. Thus, source 154 ls applied to element 144 in its poling direction and is applied to element 146 in a polarity opposed to its poling direction.
When pairs of piezo-ceramic elements are aligned and cantilevered as indicated in FIG. 6, the bimorph will bend in the direction of the element which is being driven in its poling direction. Thus, bimorph 142 bends upwardly toward element 144 when driven by source 154 with the indicated polarity. When no voltage is applied to the bimorph, there is no deflection. When a source 156 is con-nected between substrate 148 and elements 144 and 146 as shown in ~IG. 6l element 146 is driven in its poling direction and bimorph 142 deflects downwardly as indicated.

-27~

3~

For some applications, the method of driving a bimorph illustra-ted in FIG. 6, wherein a deflection voltage is applied in the poling direction of one piezo-ceramlc element and opposite to the poling direction of a second piezo-ceramic element is satisfactory. flowever, where a large amount of deflection is required, large deflection voltages are also requircd. It has been found tha-t applying large voltages in a direction opposed to the poling direction of a piezo-ceramic element tends to depolarize that element and reduce its ability to bend or deflect.
A method of driving a bimorph with large amplitude deflection voltages without depolarizing either piezo-ceramic element is illustrated in FIG. 7. In the improved method, a ; bimorph 158 has a pair of electric:ally poled piezo-ceramicelements 160 and 162 which are also aligned in a common poling ~; direction and bonded to a common substrate 164 between them.
The bimorph 158 is cantilevered at one end 166 and is free to deflect at opposed end 168. In this improved method of deflecting a bimorph, deflection voltages are applied to the piezo-ceramic elements such that the polarity of the applied voltage ls always in the poling direction of the element to which it is applied so that a large degree of deflection oE
the bimorph can be effected without depolarizing ei-ther of the piezo-ceramic elements.
~s shown in FIG. 7, when bimorph 158 is to be deflectcd upwardly, a voltage source 170 is connected between the piezo-ceramic elemen-t 160 and the substrate 164 such that the polari-ty of the applied voltage is in the pollng direction of element 160. ~o opposed polarity voltage is applied to ~35~

the element 162 since most of the bending of a bimorph is effected by the element which is driven in its poling direction.
When the bimorph 158 is to be deflected downwardly, a voltage source 172 is connected between the element 162 and the substrate 164 such that the polarity of the applied voltage is in the poling direction of the element 162. No opposed polarity voltage is applied to the element 160.
When bimorph 158 is to remain undeflected, sources 170 and 172 o~ equal magnitudes are applied between the elements 160 and 162 and the substrate 164 so that both piezo-ceramic elements 160 and 162 are driven in their poling directions. The net result of dri.vin~ both elements e~ually is that no deflection takes place.
Although the sources 170 and 172 are depicted as being constant amplitude voltage sources, they need not be.
If the bimorph 158 is to be deflected upwardly and downwardly with variable amounts of deflection, sources 170 and 172 could be made variable to accomplish such movement. Howe~er, ~0 the polarity of the voltages applied to elements 160 and 162 should always be in the poling direction of the element to which the voltage is applied.
A method of varying the magnitude and frequency of the de1ection of bimorph 158 is illustrated schematically in FIG. 8a. As shown therein, a DC voltage from a source 174 is applied to the element 160 in its poling direction. The element 162 receives a DC voltage from source 176 which is in its poling direction. ~referably, sources 174 and 176 generate ~29-~23~

positive ancl negative DC voltages respectively, of magnitudes e~ual to 1/2 Vmax~ where Vmax is the peak -to peak amplitude of the largest deflection signaJ whi.ch will be applied to the elements 160 and 162. ~lements 160 and 162 are tl-us oppositely "biased" -to 1/2 Vmax and, in the absence of any other deflection voltages, no deflec~ion of bimorph 158 will occur.
For effecting alternating deflection of bimorph 158, an AC deflection source 178 is coupled between elements 160, 162 and substrate 164 through a pair of amplifiers 180 and 182 and DC sources 174 and 176. The peak-to-peak magnitude of the AC deflec-tion signal applied in phase to elements 160 and 162 may now be as large as V without ever applying to either element a net voltage which is opposed to its poling directionO
When the deflection signal from the source 178 varies generally sinusoidally, the net voltage which appears across element 160 is indicated in FIG O ~b. With the elements 160 and 162 oppositely biased a-t 1~2 VmaX and -the superimposed ~C deflection signal appli.ed in phase to the elements, the net voltage cross each of the elements 160 and 162 always has a polarity which is in the poling direction of elements. The curves labeled "deflection" in FIG.
8b indicate that bimorph 158 de-Elects in accordance with the two times the instantaneous amplitude of the AC deflection voltage provided by source 178.
When the net voltage on element 160 becomes more (or less) positive about 1/2 Vmaxl the net amplitude oE the voltage on element 162 becomes less tor more) negative correspondingly.
However, because of the bias provided by source 176, the net ~0 voltage on the element 162 will always be in its poling direc-tion as long as the magnitude of the AC deflection voltage does no-t exceed Vmax ~JL~S~

The system shown in FIG~ 8a for driving the bimorph 158 is completely DC coupled so that bimorph 158 can be driven at very low frequencies by the source 178.
In applications where low frequency bimorph deflection is not required, a system such as that shown in FIG. 9 may be used. In the system of FIG. 9, only one amplifier 184 is needed for amplifying the AC deflection voltage from source 186. The amplified deflection voltage is applied to elements 160 and 162 via coupling capacitors 186 and 188, respectively. Separate DC bias voltage sources 190 and 192, each having an amplitude of 1/2 Vmax, bias the elements 160 and 162 so that the net voltage on either element will be in its poling direction.
Referring again to FIG. 8a,~the DC source 174 and amplifier 180 are enclosed in a dashed triangle to indicate that, in practice, they may be embodied together in one composite amplifier which ampli~ies the deflection signal and also provides the proper bias. Similarly, sources 176 and 182 may also be combined in a single composite amplifier.
An example of a pair of composite amplifiers for driving a bimorph is shown in FIG. 10. The bimorph which is being driven in FIG. 10 is part of a read transducer assembly 194 for use with the videotape apparatus shown in FIG. 3~
Transducer assembly 194 is shown schematically and in simplified form in FIG. 1~ but is preferably similar to transducer assembly 36 shown in FIG. 2. (The piezo-ceramic generator 68 is not shown as part of transducer assembly 194 only in order to simplify the drawing.) 3~

The transducer assembly 194 has a top piezo-ceramic layer 196 and a bo-ttom piezo-ceramic layer 198 bonded to a common substrate 200 which is grounded. Deflection signals are applied to the transducer assembly 194 at upper and lower conductive layers 202 and 204. Piezo-ceramic elements 196 and 198 are poled in a common direction as indicated by the arrows.
~ read transducer 199 is mounted on assembly 194 and is to be deflected in accordance with the principles and apparatus hereinbefore described. The piezo-ceramic layer 196 is driven by composite amplifier 206 and piezo-ceramic layer 198 is driven by the composite amplifier 208. The amplifiers / 206 and 208 receive low level ~C deflection signals at input terminal 210, amplify the deflection signals, and apply them superimposed on a DC bias voltage, to conductive layers 202 and 204. Generally, amplifier 206 includes a first stage of amplification provided by differential transistor pair 212 and 214 and a second stage of ampllfication provided by differentiaI txansistor pair 216 and 218. The output of transistor 218 is taken across constant current source transistor 220. The amplified signal at -the collector of transistor 218 is applied to the bases of emitter foIlowers 224 and 226 and through emi-tter resistors 228 and 230 to an output terminal 232. The signal at terminal 232 is fed back to the base of transistor 214 via a feedback resistor 234 so that amplifier 206 operates as a conventional operational amplifier with negative feedback.

~23~

The DC bias appearing at output terminal 232 is typically +100 volts and is determined b~ resistors 236, 238, the feedback resistor 23~; and the +200 volt power supply.
An AC deflection signal of 200 volts peak to peak can appear at the output terminal 232 without opposing the polarization polari,ty of piezo-ceramic layer 196. The transistors 240 and 242 provide short circuit protection for emitter followers 224 and 22~, respectively, in order to limit their output curxent in the event that terminal 232 becomes inadvertently grounded. Amplifier 208 is similar to amplifier 206 and provides an amplified deflection signal at its output terminal 244 superimposed on a DC bias of -100 volts. Amplifiers 206 and 208 can be used together to provide the amplification performed by drive amplifier 100 in FIG. 3.
The composite amplifiers 206 and 208 provide large amplitude AC deflection signals superimposed on a DC
bias voltage for driving the deflectable bimorph without depolarizing it and thereby ensure that the driven bimorph does not lose its deflection sensitivity. The transducer system shown in F~G. 10 and the mothods illustrated in FIGS.
8 and 9,and described herein forms the subject matter of the invention described and claimed in my aforementioned applicationlSerial ~o. 274,42g and provide improved per- ' formance for deflectable bimorphs.
From the foregoing, it should be appreciated that various improved bimorph devices and methods have been described which, while representing different inventions, have been disclosed toyether in the environment of an improved '' videotape read system. The bimorph motor generator combination, for example, provides a compact, reliable device rc:~

~ 3~
for sensing the instantaneous deflected position of a deflectable piezo-ceramic support arm. The illustrated embodiment of this device shows it as part of an improved videotape read assembly for generating an output signal indicative of the deflected position of a read transducer.
This novel assembly overcomes problems associated with deflectable read assemblies which vibrate when they receive an electrical or mechanical impulse by generating an output signal which can be converted to a damping signal for damping the transducer vibrations.
In accordance with the invention described herein, the damping of transducer vibrations is achieved by the described feedback control system which generates a signal indicative of the velocity of a deflected or vibrating trans-ducer, converts the velocity signal to a damping signal, and applies the damping signal to the t:ransducer support arm to dampen vibrations therein. The improved bimorph motor-generator combination is preferably used in this damping system to generage a signal indicative of instantaneous transducer position~ the transducer velocity signal being derived by differentiating the transducer position signal.
Various means are included in the damping system for stabilizing the feedback control system at frequencies near the resonant and anti-resonant points of the bimorph-motor-generator. ~his feedback control system~ in combination with the novel bimorph motor-generator transducer assembly, provides effective damping of a deflectable videotape read transducer without restricting the ~ynamic deflection range of the transducer. Moreover, this electronic damping system is not adversely af~ected by the high G accelerations normally encountered in videotape read systems.

,,~ jrc:rl3 3S~

The damping signals and transducer deflection signals are preferably applied to the bimorph transducer support arm by the method and apparatus described herein . which overcomes the depolarizing effects associated with prior methods by ensuring that the applied deflection signals are always in the poling dlrection of the piezo-ceramic element to which they are applied. A composite amplifier embodying this improved method receives large amplitude transducer deflection signals and applies them to the bimorph so as to achieve large bidirectional bimorph deflection without depolarizing the bimorph~ thereby maintaining high bimorph deflection sensitivity.

- ~S

~Z35~Z ID-2502 Figures ~ show the positionable elementin greater detail. Referring now to FIGURE // there i5 shown a mag-netic (head) transducer 311, mounted for recording and sub-sequently reading an information track upon a relatively moving recording medium. The present invention relates to a novel form of mounting structure for the head311 that permits precise, continuous positioning of the head, which structure is useful in many different types of recording environments, such as, for example, magnetic drum or disc recording, longitudinal magnetic tape recording as used for computer, audio and instrumentation purposes, transverse rotating head magnetic 'ape recording for broad band data and/or television signal recording, and helical-scan broad band data andJor television signal magnetic tape recording.
However, the structure is found to be especially suited for use in error-free positioning of heads of helical scan type magnetic tape recording/reproducing machines where large forces that act on the heads tend to promote undesirable displacements of the heads movable relative to the rotating head carrier. Therefore, the helical scan type machine as operated in a reproduce mode has been selected for illustra-tive purposes and FIGURE //shows a preferred embodiment thereof as intended for use with a single transducer. It is not intended to limit the invention to helical scanning use since the advantages of the invention in such applica-tions are also useful in other applications; however, before describing the actual invention, it will be useful to des-cribe the helical scanning structure shown in FIGURES ~
/~ and /7 and the tracking problems associated therewith, which problems the invention overcomes.

~z3~z ID-2502 Brie1y, the head3~1 can be mounted on a separate support comprising a scanning drum carrier for rotation coaxially between two stationary yuide drums, most commonly cylindrical or on a support here shown as a rotatable upper guide drum313 associated with a stationary lower guide drum 315 as in FIG~RE /~. ~ magnetic tape317 is helically wrapped ~i.e., substantially 360) around the drums313,315 for scan-ning by the head 311. The tape317 is guided, tensioned and moved (arrows319~ by means not shown but well known in the art so that the head311 carried by drum313 rotating in direction 321 opposite the direction of tape transport about the guide drums, scans a series of oblique trans~rerse paths 323 of which only one is shown in FIGURE /~ It will be seen in FIGURE that poin~325 of the tape moves to the position indicated at 327 while heaa 311 scans the tape between point 329 and point 325. The resultant path on the tape (called "track") is the line323 from point 329 to point325- The line 323 may also be termed the "direction of relative movement"
between the head311 and tape 317. In practice, the line or track 323 may be slightly S~shaped, for reasons which have nothing to do with the invention and, therefore, for simplicity of explanation the track 311 is illustrated as being straight.
It should be appreciated that if the head311 rotates in the same direction as that of the movement of the tape about the guide drums313, 315, point327 of the tape moves to the position indicated at 325 while head311 scans between point329 and point 327. Line 323' becomes the resultant track, however, this change in track position does not alter the implementation of the present invention.

~23~

As previous]y mentioned, the tape is gui~ed under tension so that recording occurs under a recommended standard value of longitudinal tension, which induces a certain degree of stretching of the tape. If the tape is played back at a dif~erent tension because of faults in the tension-ing mechanism, or because of unavoidable variations in the mechanisms of different machines, then the length, straight-ness and incllnation of track 323 will be different, and the ; head 311 will not perfectly follow the track, leading to undesirable variations in the strength of the reproduced signal and other problems. A similar effect results if the correct tension is used on playback, but the tape has shrunk or elongated due to changes in atmospheric or storage conditions, e.g., temperature or humidity. Also, irregular tape edges and differences in edge-guiding effects from machine to machine, can cause irrecJularly wandering tracks or scans.
Accordingly, the mounting of the head 311 on an extremely low-mass deflectable element is to enable it to be moved rapidly, substantially lateral to a desired track, such as a track of recorded information on a magnetic medium, while at the same time the head and its entire mounting is moved, or the recording surface is moved, or both are moved, in such a way that there is relative motion between the head and the recording surface in the direction of the desired track. This is the condition in which the head scans or follows the desired track. In one embodiment, the deflectable mounting is a thin leaf lying substantially in a plane that is normal to a plane tangent to the recording surface at the point o head-to-record surface interface and substantially parallel to the direction of relative motion.

~, r jrc:~

~ ID-2502 It should be understood that the details of the means by which the amount and direction of actual deviation from the desired track for the head is sensed, in relation to the head~to-tape path that is normally followed, and the 5 operatively associated energizing means by which the head mounting is caused to laterally deflect in response to the sensed deviations so that the head follows the desired path are not parts of the present invention, but are subject of and described in the above-mentioned co-pending applications.
Continuing now the description of the exemplary embodiment~
it will be seen from FIGURE /9 that the head 311 is fitted to the lower portion of drum313. The view of FIGURE is there-fore taken from the bottom of drum313, looking upward, as illustrated hy the arrows /~ of FIGURES /9 and /3 and the views of FIGURES ~ and /3 are also taken upside down, i.a., with the drum 313 below and th~ drum315 above, for the purpose of making the description easier to follow.
Head311 is extremely small and of low mass (on the order of lO0 milligrams), and consists of two pole pieces 33I
and 333 confronting one another across a non-magnetic transducing gap 335 for re~ording and/or reproducing signals with respect to the tape. The gap 335 is aligned with the length thereof substantially parallel to the direction321 of drum313 movement relative to the tape 317 It will be understood that in the magnetic recording art the "length" of the gap is the dimension from pole face to pole face, in the direction of relative recording motion. Usually, the "width" of gap is aligned transversely to the relative motion direction and parallel to the recording surface, and the "depth" of the gap i~

3~

normal to the recording surface. If for any reason the gap is inclined to the direction of relative motion, the length is still defined (at least for purposes of this invention) to be in the direction of relative motion, while the width and depth dimensions are still taken as being orthogonal to the length. Signals are carried to or from the head 311 by means of pole piece windings 337 and leads 328. Signals are coupled between the magnetic head311 and the recording surface passing the gap 335 through a coupling path that extends between 10 the two pole pieces 331 and 333 through the recording surface in the direction of relative motion, hence the desired track on the surface.
To provide for tracking movement of the head311 transverse (arrows339) to the direction321 of the drum 313 movement, the head is mounted or bonded, as by epoxy, to one flat side of a positioning member inc:Luding a thin deflectable le~f element 341 here shown by way of example as a piezoelectric B ceramic bender element. In the embodiment~e=~e~
discussed in detail hereinafter with reference ~o ~he drawings, the positionable element includes a cantilever mounted piezoelectric ceramic bender element ~ither manufactured by Vernitron Corp.
and identified as PZT-5HN Bender Bi-Morph Poled For Parallel Operation or by Gulton Industries and identified as ~ 1278 Piezoceramic Bender Element Poled For Parallel Operation.
As shown in greater detail in FIGURE ~ the leaf element341 is composed of two piezoelectric ceramic members 342 and343, sandwiched and bonded between electrode members (nickel or silver)349, 349A, 351 or 351Aand conductively bonded as by epoxy layers344 and 345 to opposite sides of a brass vane member 347. The ceramic 30 members 342,343 are cut and oriented with their axes of polarization vertically aligned (i.e., parallel to arrows339 in FIGURE~).

.. ... , , .. , , , , . , . . , . ~, . . . .

~23~

As is well-known in the bender art, the direction of polarization of the respective ceramic members:may be either the same or opposed, depending upon how the electrodes ~g,351 and the brass vane347, which may also be used as an electrode, are energized.
For protective purposes, the leaf341 is mounted in an open-end housing359 composed of a base shoe member 361 and a cover member363 having two side walls365 fitting on shelves~67 of the shoe361. The leaf341 is solidly mounted between two electrically insulating spacers369 by _ ,~!J _ .. , ,, . .. . , .. _ ... . .

~35~ ID-2502 means of a bolt ~71, which passes through the cover363, the leaf ~1, both spacers369, and is threaded into shoe361. The bolt371 is insulated from the leaf341 by means of an electri-cally insulating collar373 between the spacers369. To provide access to the head311 and leads338, the cover363 is made shorter than the shoe361 and is cut away in an upper slot375, the leads338 having terminals377 mounted on the upper inner end of cover363. Because a low mass is desired for the leaf 341, damping may be necessary or desirable. In such event, to provide damping and thereby lower resonant frequency for the leaf341, and to act as limit stops or restraints, the cover363 and shoe361 may be provided with so-called dead-rubber pads379,381, respectively, which absorb impact with-out immediate rebound (see also FIGURE/~). These restraints serve to prevent undesirable movement of the supported head 311 that could introduce errors in the recording and/or re-production of signals.
Leads353,355,357 extend respectively from elements 349,347,351 for coupling a voltage source to establish an energizing electric field in the elements and may be formed as shown in ~IGURE /~, in which a corner of each inwardly-extending leaf end layers349,342 and344 is cut away to leave a soldering shelf383 for attaching the lead355 to the brass vein electrode347, while the leads353 and357 are soldered respectively to electrodes349 and351. However, this arrange-ment requires a certain extension385 (FIGURE /~! of the electrodes, and in fact of the leaf341, radially inwardly of the spacers369, away from the head311. In order to pre-vent such extension385 from responding to harmonic vibrations of the drum driving motor, and other external vibration sources, ~123512 ID-2502 and thus upsetting the fine control of the movement of leaf element341, the entire extension~85 is potted between the shoe361 and cover363 as illustrated in FIGURES ~3 and ~ in which the non-conductive potting compound (e.g., epoxy) is represented by reference numeral387. The cover~63 and shoe361 may be cut away to define an enlarged potting chamber389 for this purpose.
The assembled leaf element341 and housing359 are mounted on the drum313 as shown in FIGURES // and /3 Drum 313 is provided with a cylindrical peripheral flange~91 and a central radial web393. Because the drum313 bears only one head311 as in the 360 wrap configuration, the drum web 393 and part of the flange391 are cut away to define an open-ing395 to counterbalance the mass of the head311 and its mounting means. A bracket397 is mounted in bridging relation across the opening395, as by means of bolts399.
The shoe361 is mounted on the bridging bracket397 as by means of a boltllOl, with the shoe361 extending toward the peripheries of the drums313 and315 to leave nothing 2G protruding beyond those peripheries but the tip of head311 extending through the cut away portion1103 of the flange 3~1.
For optimum performance, the dimensions and pro-portions of the leaf341 are carefully selected for the particular application intended. The leaf material is available commercially and is obtainable in various stand-ard thicknesses, which can be cut to desired length and width dimensions. The selection of dimensions and propor-tions is made according to the desired leaf element dis placement sensitivity, range and response, desired resonant -~3 -, ., .. _ , . ... . _ _ ... , . . . . .. . , . .. . .. . . . .... _ . .

35~ ID-2502 frequency, desired purity of leaf element motion, and desired structural rigidity so that the free end of the leaf element ~41 (i) is permitted to move along a desired path that re-sults in the controlled displacement of the suspended mag-netic head 11 in a direction relative to tape317 that moves the head's recording/reproducing gap335 transverse to the time axis of signals recorded along the tape and (ii) is restrained against movement that would result in the ~ap 335 of the head 11 mo~7ing in any substantial or significant manner, particularly with a component in the dlre~tion of the tirne axis, that would introduce undesirable timing errors in the recording and/or reproduction of signals. While longitudinal displacement of the free end of the leaf relative to the tape o~curs in the direction of the length dimension of the leaf as it is deflected transverse to the time axis, it does not have a significant effect in coupling signals between the tape and magnetic head. For example, in the emhodiment discribed below - including a leaf element having a leng1 h dimension, L, of 2.4 cm., the free end of the leaf moves less than 0.0001 cm. for a typical deflection of -0.024 cmO Such longitudinal displacements of the free end of the leaf do not have a compon~nt along the ~ime axis of signals recorded along the track and can be ignored for purposes of this invention. In helical scan machines, the time axis of signals recorded along the tape 317 lies along the path scanned by the head311 illustrated by line323 in FIGURE /~ More particularly, the leaf element341 should have a leng-th, L, (the suspended portion measured from spacers363 to the free leaf end at head311) to width, W, aspect ratio that restrains the element341 against any 5~2 movement in the width dimension or against any torsional movement about th~ length-width plane of the element341 that would give rise to an undesirable displacement of the suspended head311 having a component along the time axis or line323. Undesirable displacements that are to be particu-larly avoided are those that would introduce unacceptable azimuth and time base errors in the recording and reproducing of signals. Por signals in the color television video fre-guency range, displacement along the time axis or line323 should be limited to less than 0.13 microns in order to avoid such errorsO On the other hand, it is preferred that the length-to-width aspect ratio not be so small as to unculy . _ . .. . , .. _ . _ , _ . . _ .. ..

~23~

1 limit the possible head displacement range for a practic-al drive voltage used to control the displacement of the element 341. For example, for a head displacement range of ~ 0.025 cm., a length-to-width aspect ratio of 2 is the most suitable.
As the aspect ratio is increased, the leaf element341 becomes less rigid in the width dimension and, eventually, is able to move in a direction having a component along the time axis or line323 causing unacceptable azimuth and time base errors.

As the aspect ratio is decreased, the leaf element341 does become more rigid in the width dimension. But, the drive voltage must be increased for a given head displacement, eventually to levels that become impractical, particularly, for the rates of displacement cycles necessary to maintain the error-free tracking that the present invention is intendea to provide for helical scan applications.

The thickness, t, of the leaf element341, is selected, in the preferred embodiment described herein, to provide good sensitivity, i.e., displacement per unit drive voltage, sufficiently high resonant frequency to permit the elemen~341 to ~e displaced at desired high rates below the resonant frequency, purity of leaf element motion and a practical voltage limit for the desired maximum displacement rate and range. For example, for a displacement rate of up to about 200 displacement cycles per second over a range of ~ 0.025 cm., a thickness on the order of 3% of the width dimension of the element3~1 is suitable. ~hile leaf elements of smaller thicknesses are characterized by greater sensitivity, they also have a lower resonant frequency. As the rate of leaf displacement approaches a resonant frequency, the leaf displacement exhibits marked changes from displace-ments at frequencies either side of the resonant frcquency.

~L~23~
Such marked displacement ehanges make control of the of the position, hence tracking of the leaf element 341, exceedingly difficult. The opposite is the case for leaf elements of greater thickness, i.e., decreased sensitivity and higher resonant frequency. Further, thicker lead elements require higher drive voltages for a desired displacement range and rate. Torsional displacements giving rise to unacceptable time base and azimuth errors are futher restrained by constructing the leaf element 341 to experience a pure bending motion type displacement when subjected to an energizing electric field. Such displacement is achieved by constructing the leaf element 341 to have a uniform thickness over its length. A thickness uniformity along the leaf's length of + 10% of the thickness design value provides exeellent restraint against unacceptable torsional displacements.
The positionable head mounting structure is further characterized as being eapable of a very low mass (1.5 grams is a typical example) eonstruction. The low mass construction is possible because the structure utili2es a single thin leaf positionable element 3~1, from which is suspended a magnetie head 311 o~ rela-tively ne~ligible mass. The low mass characteristie of the strueture faeilitates-the ~apid displacement of the head 311 ~ 47 .
jre:~
;

~ 3~

under carcfully controlled cond~tions whereby it can be precisely positioned to follow a desired path along the magnetic tape317. Furthermore, it enables the positionable head mounting structure to be used in rotary scan recordt reproduce machines, such as helical scan machines of the kind in current commercial use.
In one embodiment of the posi~ionable head mounting structure used in a helical scan machine, the leaf element was constructed to have a thickness, t, of 0.05 cm., and an extension (or length, L,) dimension of 2.4 cm. in order to provide a resonant frequency of about 400 deflec~ion cycles per second. The wid-th of the leaf element341 was selected to be 1.27 cm., a value that provided àdequate stiffness or rigidity in the direction of the scan of the head311 over the tape317 (or time axis of the signal recorded along the tape), considering the frictional drag created by the tape, and the repeated ex~remely large i.mpulse change in the frictional forces acting on the head311 as it enters and leaves each scan of the tape317. Particularly to be avoided is an effect of twisting of the leaf about its longitudinal axis, which would cause a skewing effect of the head with respect to the tape. The dimensions selected were found satisfactory to avoid skew.
For some applica~ions, it may be desirable to moun~
~5 a plurality of magnetic transducers on the positionable element. For example, FIGURE /~illustrates an application in which a pair of left-offset and right-offset t~ck sensing magnetic heads1105 and1107 are employed to monitor . , , . . . . ..... ......... . . ....... .... , , ... . . . _ _ , , , . _ _ . , ~ _ .

3rD3.~
continuously the position of a single record/reproduce magnetic head 311a relative to a recorded track and provide information that is used to control the poCition of the record/
reproduce head. The implementation of this embodiment for controlling the position of a single record/reproduce head is descrlbed more fully in my above-referenced co-pending application, Serial No. 274,280. The single record/reproduce head 311ais mounted just as is head 311, while track sensing heads 1105, 1107 are mounted in either side of head 311a, but are oppositely staggered transversely to the direction of motion 321a, so as to sweep, respectively, left-offset and right-offset zones 1111 and 1113 that overlap the middle zone 1115, which corresponds to the expected range of track displacement of head 311a. As shown in FIGURE 17 record/reproduce head 311a is mounted directly on the surface of the lead 341a for sweeping a range of displacements represented by middle zone 1115. Left-offset track sensing head 1105 is mounted on a spacer element 1109 fastened to the surface of the lead 341a, the thickness of the spacer 1109 being less than the width of the head 311a so that the sensing head 1105 is spaced above the head 311a by an amount less than the width of the head 311a. Right-offset track sensing head 1107 is mounted on a recessed mounting shelf 1117 provided by cutting away lead 341a at the corner, somewhat as in FIGURE 16. Mounting shelf 1117 is recessed below the surface of leaf 341a a distance e~ual to the thickness of the spacer llOg so that the sensing head 1107 i5 spaced below the head 311a by an amount less than the width of the head 311a. With the track sensing heads 1105, 1107 irc~

~1~3~2 ID-2502 mounted in thc aforcdescribed manner relative to the rccord/
reproduce head311a, the paths scanned by the sensing heads always overlap the edges of the path scanned by head311a as it is displaced through the expected ranye1115 of track displacement. In the event the path scanned by the head311a is a recorded track of information, the sensing headsllos~
1107 reproduce information from the overlapped edges of the recorded tracks as they follow the record/reproduce head311a.
Alternatively, the sensing headsllO5,1107 may be made narrower in width (i.e., transverse to direction of motion321a) than head311a, so as to have less overlap upon the path scanned by head311a, or even zero overlap. ~lowever, the headsllO5,1107 preferably do not extend laterally beyond the dimension of the guard bands flanking the recordeu track, when the head 311a is correctly following the track, and thus heads1105,1107 do not ordinarily read parts of adjacent tracks. With regard to other structural features of t:he transducer mounting structure of FIGURE 1;~, such as, for exampIe, a housing, head windings, electrical leads, and restrains, they may be con-structed similarly as described with reference to the ~embodiment of FIGURES /~ through ~.
FIGURES ~/~ and ~7~B illustrate, in schematic block diagram form, embodiments of means for sensing the position of the record/reproduce head relative to a desired path along a record surface, such as a recorded track of information, and generating a su.itable signal for actuating the positioning element by, for example, energizing the piezoelectric member ~42 and343 for displacement to control the position of the head so that it follows the path or recorded track. The embodiment of F:rGVRE o~ 9 is for use with the magnetic transducer mounting structurc cmbodiment illustrated by FIGI~RES /~ through /~ and ~,2 ;3~
utilizes a dithering technique to sense and control the position of the record/reproduce head 311. The embodiment of FIGURE 21B is for use with the magnetic transducer mounting structure embodiment illustrated by FIGURE 17 and utilizes a track following technique described in detail in my above-referenced co-pending application, Serial No. 274,280 to sense and control the position of the record/reproduce head 311a. Considering first the position sense and control embodiment of FIGU~E 21A as employed with the mounting structure embodiment of FIGU~ES 10 through 16, an oscillator 1151 is operated to provide at its output a fiYed frequency alternating dither signal, which is coupled to the lead element 341 causing it to vibrate within a displacement range. Before coupling to the leaf element 341, the dither signal is coupled to one input of a voltage summing circuit 1152 to be algebraically summed with a voltage control signal provided by an adjustable bias voltage source 1153 and coupled to a second input of the summing circuit. The resulting summed dither and control signal provided at the output of the summing circuit 1152 is coupled by line 154 to be applied between the two leads 353 and 357 so that the summed signal is impressed across the entire leaf element str~cture. If the summed signal is to be applied to leaf element 3~1 with reference to the brass vein electrode 347, the other electrode 355 is required. One of the electrodes, for e~ample, 351 connected to the lead 357, serves as a reference for the applied summed signal.
The dither signal component of the applied summed signal causes the leaf element 341 to vibrate over the jrc:~V~

1 ~235~2 ID-2502~

selected range as the suspended head 11 is operated to reproduce signals recorded along the track, such as repre-sented by line323. This vibration causes an amplitude modulation of the envelope of the reproduced signal. When head~ll is located in the proper track position at the center of the track323, the amplitude modulation of the reproduced signal at the dither frequency is at a minimum and increases to a maximum as the head 311 is displaced to one side or the other of the track center. Thus, minimum peak-to-peak values of the signal envelope at the dither frequency occur when the head311 passes through track center and greater peak-to-peak signal envelope values at the dither frequency occur when the head311 is displaced to one side or the other of the track center. With the head311 in the proper track position, the frequency of the envelope variation is twice the frequency of the dither signal component. However, with the head311 to either side of the proper track position, the maximum-to--minimum envelope amplitude variation occurs once for each cycle of the dither signal component, or at the dither signal frequency, with the order of occurrence of the maximum and minimum points depending upon the side of track center to which the head311 is offset. Detection of the order of occurrence of the maximum and minimum points provides infor-mation definitive of the direction the head311 is offset from the center of track323 and detection of the envelope ampli-tude variation provides information definitive of the amount of offset.
To obtain this track offset information, leads338 of the head311 are coupled to the input of an envelope detector 1156. The de-tector provides a signal 3~

representative of the amplitude modulated envelope component of reproduced signal at the frequency of the dither signal.
This signal is coupled to a control input of syn~
chronous detector1157 for phase and amplitude comparison , with the dither signal provided by the oscillator1151 and coupled to a reference input of the detectorll57. The det~ctor1157 is responsive to the input signals to generate an output signal having an amplitude proportional to the amount head311 is offset from track center and a polarity representing the direction of the offset. This output signal is provided to the input of the ad~ustable bias voltage sourcell53 to adjust the voltage level of the control signal in accordance with the amplitude and sense of the output signal. Source 1153 is responsive to the output signal to generate a control signal whose voltage level follows the ar.lplitude and sense variations of the output si~nal so that the positioning leaf element341 is energized to compensate for de~ected trac~.
offsets of the head~ll upon application of t~e summed control signal and dither signal.
l~ith reference to the track followin~ e~bodiment o FIGURE ~7/~ as employed with the transducer mounting structure embodiment of FIGURE /~ it includes an adjustable bias voltage sourcell61 that provides a control. signal at its output, which is coupled by linell62 to leads353a and357a to be applied, as in ~he embodiment of FI~,~TRE ~/~ across the entire leaf element structure341a. Two inputs of a diference detector 1163 are respectively coupled to receive the signals repro-duced by the sensing heads1105,1107. The difference detector - S~

~ ~ ~ 3 ~ 2 1l~3 compares the average amplitudes of the reproduce(l si~nal envelopes and provides an output diference signal whose amplitude is proportional to the difference in the average amplitudes and whose sense i5 representative of which of the average amplitudes is the largest. ~Then head311a i5 located in the proper track position at the center of the trac~323, the average amplitudes of the signals reproduced by the sensing headsllO5,1107 are equal. Thus, the output signal of the difference detector will be zero, or correspond to ~he desired track position for head311a. ~lowever, as the head311a is displaced from track center in the direction of the lef~-offset track sensing `headllO5 (see FIGUP~ /~)~ the average amplitude of the signal envelope reproduced by the sensing headlln5 proportionately decreases while that reproduced by the right-ofset trac~; sensing headllO7 proportionately increases. The contrary occurs as the record/reproduce head~lla is displaced from tracl; ~enter in the direction of the right offset track sensing headllO7, i.e., the average amplitude of the signal envelope reproduced by the sensing headllO7 proportionately 2n decreases while thflt reproduced by the sensln~llO5 proportionately increases. The difference detectorll63 is responsive to such proportionately ch2nging signals to generate a difference signal whose amplitude follows ~he ampli~ude difference OL
the signal er.velopes reproduced by the sensing heads1105, 1107 and whose sense is dependent upon which of the sip,nal envelopes has the greatest average amplitude. This di.fference signal is provided to an input of the adjustable bias voltage sourcell61, ~hich is responsive to adjust the voltage level of the control signal in accordance with the amplikude and sense of the difEerence si~nals so that, upon its application

2 In-2502 ~o the positioning leaf element~41a, the element is energlzed to compensate for detected trac~ offsets of the head311a.
An alternative arran~,ement for mounting the trans-ducer is shown in FIGURE ~O. In this exan-'ple, the leaf elementll21 is not piezoelectric but is made of magnetically-permeable material, and is arran~ed to pivot from a stable support, rather than bend, provided by means of a pair of widely-spaced knife-edge type hingesll23 for~.ed bet~een ~he leaf ~21 and a base mem~erll25 with the leafll21 loaded -against a basell25 by means of a compression sprlng element 1127 extending between the leaf and the ~ase. Head llb is mounted at the end of the leafll21. The basell25 also includes a pair of electromagnetsll29 ~ositioned, by suitable retaining means (not shown), on opposite sides of the leaf for p~oducing a magnetic ield thrcu~h the leafll21 in a directionll31 (or1133) that is normal to the plane of the leaf. ~rive means 1135 for energizing the electromagnetics to position the leaf 1121 are sche~atically shown.
The embodiment illustrated by FIG~IRE ~o utilizes a dithering technique like that described with reference to FIGU~E ~ for controlling the position of the leafll21 at its head end. More specifically, the leaf1121, and its pivoted support structure, is made of magrLetically permeablc material.
The drive meansll35 includes a current sourcell37 that delivers over linesll39,1141 a su~.med dither and control current signal to the exciting coils of the electromagnetsll29. For con-venience, the windings of the coils are wound about the cores of the electromagnets in opposite phase senses so that opposite magnetic poles are established at the facing surfaces of the cores. This permits the same phased current signal to be used ~ 3~ ID-2502 .
for exciting hoth coils to control and vary the po~sition of the leafll21.
~ s in the embodiment of FIGURE ~/~ an oscillatorll~3, detector and bias sourcell45 and summing circuitll47 are operatively associated together and coupled to receive the signal reproduced by the head311b and generate a summed dither and bias control signal for application to the control input of the current source~137. The oscillatorll43 generates the fixed frequency alternating dither signal for exciting the electromagnet coils to vibrate the leaf1121 within a determined displacement range. The bias control signal determines the current level about which the current signal provided by sourceL137 is made to vary at the dither signal frequency and has an amplitude determined by the amplitude variation a-t the dither frequency of the signal envelope reproduced by the head 311b and by the order of occurrence of maximum and minimum envelope amplitude points.
While the transducer mounting of the present invention has been described particularly in relation to magnetic helical scan applications, it will be apparent that the positionable transducer mounting is equally well adapted for use with other signal recording systems employing transducers other than magnetic heads. Also, other types of record medium scanning apparatus may be used, such as transverse scan apparatus, magnetic discs and rnagnetic drums, and logitudinally recorded tapes. For transverse scan, the head, or an appropriate number of them, may be mounted in a similar manner on the scanning drum. In -~
the magnetic drum and disc art, the mounting is well adapted to enablé the head to ~ollow apparent track irregularities that may be caused by wobble or run-out, such as may, in turn, be caused by eccentric or axially misaligned drums/discs . ~ ~

~ ~ ~ 3 S~2 ~D-2502 or mis-allgnment of the head movlng mechanism. In longitudinal recording, the head mount of the invention permits the head to follow apparent track irre~ularitles such as may be caused by mis-alignment of the tape guides or head mounting base, or simply by wavy tape edges engaging well-aligned guides when the tape has shrunk or expanded after having been recorded.
For parallel channel recording applications, more than one record/reproduce head can be supported from a single positioning element, ~lat has been described is the adaptation of a magnetic transducer to automatic tracking use in association with a relatively moving magnetic recording sur~ace such as a magnetic tape, drum or disc, the transducer being supported from a positioning element for displacement lateral to the time axis of signals recorded along the record surface, commonly, the direction of relative motion with respect to `the record sur~ace, while restrained against deleterious displacement along the time axis. For applications in which the transducer is to Eollow a previously recorded track, the transducer is displaced with a predeter-mined range corresponding to the expected range of track deviation on the record surface.

Claims (11)

The embodiments in which an exclusive property or priv-ilege is claimed are defined as follows:

1. Damping apparatus for damping vibrations in a de-flectable videotape transducer, comprising: a transducer support arm for supporting a videotape transducer and for deflecting the transducer in response to an applied electrical deflection sig-nal, said deflectable support arm comprising a piezoelectric bimorph having first order resonance characteristics and higher order resonance characteristics; signal generating means for generating a deflection velocity signal representative of the instantaneous deflecting velocity of the transducer; a negative feedback loop coupled between the output of said signal gener-ating means and said support arm and including means for con-verting said deflection velocity signal to a damping signal and means for applying said damping signal to said deflectable sup-port arm to dampen vibrations therein, said feedback loop in-cluding a filter for excluding from said damping signal the com-ponent of said deflection velocity signal attributable to said higher order resonance characteristics.

2. Deflecting apparatus as set forth in Claim 1 where-in said bimorph includes sensing means integral to said bimorph for generating a signal representative of the instantaneous de-flected position of said transducer.

3. Deflecting apparatus as set forth in Claim 2 where-in said bimorph support arm has a cantilevered end and an opposed deflectable end and said sensing means comprises a piezoelectric generator extending substantially from the cantilevered end por-tion of said bimorph to the deflectable end portion of said bi-morph.

4. Damping apparatus as set forth in Claim 1 wherein said feedback loop includes phase compensation means for compen-sating for phase shifts between said electrical deflection signal and said deflection velocity signal so as to stabilize said feed-back loop from oscillations due to large amplitude vibrations of said bimorph at its first order resonance.

5. In a videotape recorder having a read transducer for reading information from tracks of information on a video-tape and receiving electrical deflection signals for deflecting said transducer to the center of the track being read, transducer damping apparatus for damping vibrations in said transducer, com-prising: a bimorph support arm having a cantilevered end portion and an opposed deflectable free end portion at which it supports the read transducer, said support arm deflecting in response to deflection signals for positioning the read transducer at the center of the track, said bimorph having a frequency response exhibiting first order resonance and anti-resonance characteris-tics and higher order resonance characteristics; sensing means for generating a position signal corresponding to the instanta-neous deflected position of said read transducer; a negative feed-back loop coupled between the output of said sensing means and said deflectable support arm including means for converting said position signal to a damping signal and for applying said damping signal to said deflectable support arm to damp vibrations therein;
and means for adding a portion of said deflection signal to said position signal to reduce the gain of the feedback loop at fre-quencies near the first order anti-resonance of said bimorph.

6. Transducer damping apparatus as set forth in Claim 5 wherein said sensing means is integral to said bimorph.

7. Transducer damping apparatus as set forth in Claim 5 wherein said feedback loop includes filter means for excluding from said damping signal components of said position signal attrib-utable to the higher order resonance characteristics of said bi-morph.

8. Transducer damping apparatus as set forth in Claim 7 wherein said feedback loop includes phase compensation means for compensating for phase shifts between the deflection signal and the position signal and for phase shifts introduced by said filter means at frequencies near the first order resonance of said bimorph.

9. Damping apparatus for damping vibrations in a de-flectable videotape read transducer comprising: a bimorph sup-port arm having a cantilevered end portion and an opposed free end portion at which it supports the read transducer, said bi-morph having a frequency response exhibiting first order reso-nance and anti-resonance characteristics and higher order reso-nance characteristics; sensing means for generating a position signal corresponding to the instantaneous deflected position of said read transducer; a negative feedback loop coupled between the output of said sensing means and said deflectable support arm, including means for converting said position signal to a transducer velocity signal, filter means for excluding components of said position signal attributable to the higher order reso-nance characteristics of said bimorph, phase compensation means for compensating for phase shifts between the deflection signal and the position signal and for phase shifts introduced by said filter means at frequencies near the first order resonance of said bimorph, and a negative feedback amplifier for receiving and inverting the phase compensated velocity signal and applying its output to said deflectable support arm for damping vibrations therein.

10. In a videotape recorder having a read transducer for reading information from tracks of information on a videotape and receiving electrical deflection signals for deflecting said transducer to the center of the track being read, transducer damping apparatus comprising: a bimorph support arm having a cantilevered end portion and an opposed free deflectable end portion at which it supports the read transducer, said support arm deflecting in response to deflection signals for positioning the read transducer in the center of a track, said bimorph having a frequency response exhibiting first order resonance and anti-resonance characteristics and higher order resonance characteristics; sensing means for generating a position signal corresponding to the instantaneous deflected position of the read transducer, said sensing means being integral to said bi-morph and comprising a piezo-ceramic generator extending substan-tially from a cantilevered end portion of said bimorph to the free end portion of said bimorph; a negative feedback loop coupled between the output of said sensing means and said de-flectable support arm, including means for differentiating said position signal to convert it to a transducer deflection veloc-ity signal, filter means for excluding from said transducer ve-locity signal components which are attributable to said higher order resonance characteristics of said bimorph, phase compen-sation means receiving the output of said filter means for com-pensating for phase shifts between said electrical deflection signal and said transducer velocity signal so as to stabilize the feedback loop from oscillations due to large amplitude vi-brations of said bimorph at its first order resonance, a nega-tive feedback amplifier receiving the output of said phase com-pensation means for inverting the phase compensated velocity signal, and a deflection amplifier for applying the output of said feedback amplifier to the deflectable support arm for damp-ing vibrations therein; and means for adding a portion of said deflection signals to said transducer position signal to reduce the gain of the feedback loop at frequencies near the first order anti-resonance of said bimorph.

11. Damping apparatus for damping vibrations in a de-flectable transducer, comprising: a transducer support arm for supporting a transducer and for deflecting the transducer in re-sponse to an applied electrical deflection signal; signal gener-ating means for generating a positive signal representative of the instantaneous deflected position of the transducer; a nega-tive feedback loop coupled between the output of said signal gen-erating means and said support arm and including means for con-verting said position signal to a damping signal and for applying said damping signal to said support arm to dampen vibrations therein; and means for adding a portion of said applied electrical deflection signal to said position signal to reduce the gain of the feedback loop at frequencies near the first order anti-reso-nance of said transducer.